This application claims the priority benefit of Taiwan application serial no. 88119200, filed Nov. 4, 1999.
1. Field of Invention
The present invention relates to an optical fiber grating. More particularly, the present invention relates to an optical fiber grating capable of filtering light wave around a central wavelength with position of the central wavelength being adjustable.
2. Description of Related Art
Following rapid progress in high-density wavelength division multiplexing techniques for optical transmission, the ability to extract or insert signals at a definite wavelength at a particular node point become increasing important. To increase the variability of transmission capacity, optical designs with wavelength regrouping capability is in great demand.
Bragg type of optical grating has an optical core whose refractive index varies cyclically. Cyclically varying refractive index is generated by crossing two ultraviolet light beams so that interference lines are formed within the optical fiber. Alternatively an ultraviolet light beam is shone on a photomask to produce the interference lines in the optical fiber. Due to the photosensitivity of optical fibers, variation in refractive index is achieved.
When an incoming light beam transmitted through an optical fiber grating and if one of the wavelengths within the incoming light beam satisfies the Bragg condition, that particular wavelength will be reflected (in other words, same as the central wavelength of the optical grating). In general, the central wavelength reflected from the optical grating is twice the average effective refractive index times the period of refractive change. On the other hand, wavelengths that do not satisfy the Bragg condition will just pass through the optical grating.
As temperature around the optical grating changes, corresponding changes in the refractive index will lead to a shift in the central wavelength of reflection. For an optical grating that operates at a wavelength of 1550 nm, the wavelength/temperature coefficient is about 0.012 nm/xc2x0 C. In other words, for every 1xc2x0 C. change in temperature, the central wavelength will shift by 0.012 nm.
In addition, forces acting on the optical grating change, both the refractive index and the period of fiber grating will be affected leading also to a shift in the central wavelength of reflection. For an optical grating that operates at a wavelength of 1550 nm, the wavelength/strain coefficient is about 1.2 nm/millistrain.
Due to possible strain and temperature changes, consideration must be made with regard to the application of the optical grating. For example, in U.S. Pat. No. 5,694,503, the optical grating is fastened onto a ceramic plate that has a negative coefficient of thermal expansion. Furthermore, in U.S. Pat. No. 5,841,920 and World Intellectual Property Organization Patent No. WO9827446, an assembled structure consisting of two components having very different coefficient of thermal expansion are used to produce a negative coefficient of thermal expansion. Both types of inventions are applied to an optical grating working at a fixed wavelength. Strain produced by temperature change in the ceramic plate or the assembled structure is capable of compensating for the drift in central wavelength. In other words, drift in wavelength is minimized when temperature around the optical grating changes, or equivalently, the wavelength/temperature coefficient is reduced.
In addition, methods of adjusting the central wavelength of an optical grating are also developed. For example, in U.S. Pat. No. 5,812,711, changes in magnetic strain is used to adjust the operating wavelength of the optical grating. In U.S. Pat. No. 5,469,520, the optical grating is strained to adjust the operating wavelength. On the other hand, in European Patent No. EP0867736, the optical grating is heated to adjust the operating wavelength.
As the number of multiplexing channels is increased, channel spacing has been reduced from about 1.6 nm to just under 0.4 nm. Consequently, position of the central wavelength must be very precise and tolerance of the central wavelength has to be lower than xc2x10.025 nm. If the heating method is used to adjust operating wavelength, power for heating up the grating has to be constantly supplied. If piezoelectric material (for example, lead zirconium titanium oxide PbZrTiO3 or PZT) is used to adjust operating wavelength, constant power supply is also needed. If the magnetic flux method is used to adjust operating wavelength, although power is needed only when adjustment is required, consideration must still be made regarding the wavelength/temperature coefficient and the thermal expansion coefficient of the magnetic material. Finally, if operating wavelength is adjusted by straining the optical fiber in the axial direction, the resolution of the mechanic has to be in the micrometer range. Therefore, demands in manufacturing precision are exceptionally high.
In all the aforementioned methods of adjusting the central wavelength of an optical grating, drift in central wavelength due to a temperature change is still ignored. In other words, even if the operating wavelength has been precisely set, any changes in temperature may still cause some undesirable drifting.
Accordingly, one object of the present invention is to provide an optical fiber grating that uses a bimetallic strip and a compression spring to position the central wavelength of reflection for an incoming light beam. In addition, the grating is capable of offsetting any drifting of the central wavelength due to a temperature change.
A second object of this invention is to provide an optical fiber grating whose central wavelength of reflection can be adjusted yet quite insensitive to temperature fluctuation. Hence, there is no need for monitoring surrounding temperature and performing feedback control actions constantly.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides an adjustable light-reflecting component. The light-reflecting component includes a waveguide that constrains and transmits an incoming light beam. The light-reflecting component is incorporated with the waveguide for reflecting incoming light signals around a central wavelength of the light-reflecting component. The light-reflecting component is fastened onto one side of a bimetallic strip. Any deformation of the bimetallic strip will cause some strain in the light-reflecting component and lead to a shift in the central wavelength of the reflected light signals. In addition, differences in coefficient of thermal expansion between the two metallic strips of the bimetallic strip will produce bending stress that can almost compensate for any drift in central wavelength when the temperature changes. One end of an elastic prop is attached to one side of the bimetallic strip while the other end is attached to a drawing point. Therefore, the bimetallic strip is bent by a force transmitted through the elastic prop.
The invention also provides an adjustable optical fiber grating system. The system includes a bimetallic strip formed by joining the flat surfaces of a first and a second metallic strip respectively. The coefficient of thermal expansion of the first metallic strip is smaller than the second metallic strip. The system also includes an optical fiber that incorporates an optical grating for reflecting incoming light signals around a central wavelength. The optical fiber grating is fastened onto one side of the first metallic strip so that any deformation of the bimetallic strip will cause some strain inside the grating and ultimately will result in a shift in the central wavelength. Furthermore, differences in the coefficient of thermal expansion between the two metallic strips will result in a bending stress in the bimetallic strip capable of compensating almost any drift in central wavelength due to a temperature change. One end of an elastic prop is in contact with one side of the second metallic strip while the other end is attached to a drawing point. Hence, the bimetallic strip is bent by a force transmitted through the elastic prop.
The invention also provides a method for adjusting an adjustable light-reflecting component. The method includes the steps of providing a waveguide having a light-reflecting component capable of reflecting incoming light signals around a central wavelength. A bimetallic strip with the light-reflecting component attached to one surface of the bimetallic strip is next provided such that any deformation of the bimetallic strip will cause some strain in the light-reflecting component. Furthermore, deformation in the bimetallic strip due to a temperature change can be used to offset any drift in the central wavelength of reflection due to the temperature fluctuation. Finally, an elastic prop is provided for exerting a force on the side of the bimetallic strip so that the resulting deformation in the bimetallic strip can be used to adjust the central wavelength of reflection for the light-reflecting component.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.